This invention generally relates to brightness controls for display devices. More particularly, this invention relates to automatic brightness control systems for display devices utilizing brightness adjustment.
Display devices are used in a variety of consumer and industrial products to display data, charts, graphs, messages, other images, information, and the like. Backlight display devices, which may be backlit or frontlit, have a backlight positioned to provide light for a display panel. Emissive display devices have pixels that are the emissive light source. In emissive displays, the pixel light source may be a CRT phosphor, a FED phosphor, a light emitting diode (LED), an organic LED, an electroluminescent, or any emissive display technology. In backlight display devices, the backlight may be a fluorescent tube, an electro-luminescent device, a gaseous discharge lamp, a plasma panel, LED, and the like. The display panel may be a light emitting diode (LED) and may be a passive or active matrix liquid crystal display (LCD). The backlight and display panel are connected to control circuitry, which is connected to a voltage supply. The display device may be separate or incorporated with other components, such as a dashboard in an automobile or other vehicle, a portable electronic device, and the like.
Generally, the brightness of the display panel is controlled in relation to the environment of the display device and user preferences. A poorly lit environment usually requires less brightness than a brightly lit environment. Also, a brightness level suitable for one user may not be suitable for another user. In a typical display device, a user adjusts the brightness manually. There may be a switch, a keypad, a touch screen, a remote device, or the like to adjust the brightness. The brightness usually remains at the fixed level until the user changes the level.
A fixed brightness level may be suitable when there is consistent ambient light during operation of the display device or when a user need only make an occasional adjustment. However, in many applications, a fixed brightness level may not be suitable and may not be desirable. Ambient light seems to constantly change or changes very frequently in many applications such as automobiles and other vehicles or in portable applications. There are the extremes of night and day and in-between conditions such as dusk and dawn. Other in-between conditions include brightly lit highways at night and tunnels during the day. There also are differences in the ambient light on cloudy and sunny days. The changes in ambient light conditions may similarly affect other applications using backlight display devices, such as cellular telephones and other communication devices, personal organizers, laptop and personal computers, other portable electronic devices, and the like. Some applications use a display device in various locations having different ambient light conditions. These locations may include an office, the outdoors, inside a vehicle, and the like. The different ambient light conditions usually require adjustments to the brightness level for comfortable viewing of the display device. Additionally, automatic adjustments to the brightness level may improve battery consumption, improve light source life, and minimize image burn-in (image retention) such as occurs with emissive displays such as organic LEDs, plasma, FEDs and the like.
In automotive applications, one approach is to reduce the brightness of a backlight display device when the headlights are switched-on. A user may further adjust the brightness manually. There essentially are two brightness xe2x80x9clevelsxe2x80x9dxe2x80x94a first level when the lights are switched-off and a second, lower level or range when the lights are switch-on. However, this approach does not automatically change the brightness in relation to changing ambient light conditions. Additionally, there may be unsuitable brightness levels for particular ambient light conditions. The brightness level may be too low at dusk or on a cloudy day when the lights are switched-on. Generally, the two-level and manual adjustment is not well suited for uses of backlight display devices in automotive applications, especially those devices conveying large amounts of detailed information such as maps and other navigation features, internet messages, other communications, and the like.
In addition, brightness adjustments that are exponential in nature provide less luminance change at lower display luminance levels and more luminance change at higher display luminance levels. The logarithmic nature of the human eye perceives equal luminance step ratios as equal brightness steps. To a user, a luminance change from about 10 Nits to 12 Nits (a ratio of about 1.2) appears like the same luminance change from about 100 Nits to 120 Nits (a ratio of about 1.2). A Nit is a unit of luminance for light reflected, transmitted, or emitted by a surface, such as a display panel. Brightness adjustments that do not correspond to the perception capability of the eye often result in different brightness levels than what is needed or desired. For example, a display device may have a daytime brightness range from 50 to 450 Nits. A brightness control system which linearly increases the brightness as a function of a control device such as a potentiometer or brightness step controller would be too sensitive for low brightness levels and not sensitive enough for the higher brightness levels. This brightness range may have about 8 steps, with each step increasing the brightness by about 50 Nits.
In contrast, brightness control systems that control the display luminance as a function of ambient light and in relation to the preferred human eye transfer function are different in comparison to manual adjustment requirements. The function is essentially a straight line on a log-log scale where the ordinate (Y axis) is the emitted display luminance and the abscissa is the reflected ambient light from the display. The slope and offset of the straight-line transfer function on the log-log scale is a function of the display type. Automatic brightness control systems that do not follow this function may provide too little or too much brightness than what is needed for comfortable viewing. If the brightness is too little, the user may not be able to see the display device. If the brightness is too much, the xe2x80x9cexcessxe2x80x9d brightness may provide an uncomfortably bright display. The excess brightness increases the power consumption, reduces efficiency, and increases the operating costs of the display device. The excess brightness also reduces the operating life of the display device and will accelerate image burn in for emissive type display devices. Generally, the higher the brightness and the longer the time at a higher brightness, both tend to reduce the operating life of the display and for portable devices increases the battery consumption rate. During direct sunlight or similar ambient light conditions, the brightness level is set at or near the maximum brightness level for a user to see the display device. However, the brightness level does not need to be at the maximum level all the time as may be the case in diffused ambient light conditions where the sunlight is not directly impinging on the display.
Many automatic brightness control systems frequently attempt to use a linear method that proportionally changes the display brightness as a function of the sensed ambient light. Such a system may have a lower than desired display luminance except at the end points and may be especially dim at the lower ambient lighting levels. This is because the user desired straight line transfer function on a log-log scale is a fractional power function which requires that the brightness to increase rapidly at lower ambient light levels and then increase more slowly as the ambient light level increases to a maximum level. The linear adjustments also may provide too little brightness than what is needed for comfortable viewing.
This invention provides an automatic brightness control system for a display device. The brightness control system adjusts the display luminance of the display panel as a fractional power function of the ambient light impinging on the display panel. The manual brightness or luminance adjustments, including preference offsets to the automatic brightness control transfer function on a log-log plot, have essentially constant ratio steps, enabling a user to perceive the adjustments as equal brightness changes. By implementing a user preference offset adjustment using constant ratio luminance steps, the automatic brightness control system may be adjusted in a manner to suit one or more viewing preferences.
In one aspect, a display device having an automatic brightness control system may have a lighted display, a sensor, and control circuitry. The sensor is disposed to logarithmically sense ambient light near the lighted display. The control circuitry is connected to receive a first signal from the sensor. The control circuitry also is connected to provide a display luminance to the lighted display. The control circuitry selects the display luminance from at least one luminance adjustment sequence. Each luminance adjustment sequence has a plurality of luminance values with constant ratio steps. The display luminance is a fractional power function of the ambient light near the lighted display.
In another aspect, a display device having an automatic brightness control system may have a lighted display, a sensor, a user interface, and control circuitry. The sensor is disposed to logarithmically sense ambient light near the lighted display. The control circuitry is connected to receive a first signal from the sensor and connected to receive at least one user selection from the user interface. The control circuitry also is connected to provide a display luminance to the lighted display. The control circuitry selects the display luminance from at least one luminance adjustment sequence. Each luminance adjustment sequence has a plurality of luminance values with constant ratio steps. The display luminance is a fractional power function of the ambient light near the lighted display. The fractional power function is adjusted by a constant ratio offset based on the at least one user brightness selection.
In one method for controlling the brightness of a display device, a first signal is generated in response to the ambient light associated with a lighted display. A display luminance is selected from one or more luminance adjustment sequences. Each luminance adjustment sequence has multiple luminance values with constant ratio steps. The display luminance is provided to the lighted display. The display luminance is a fractional power function of the ambient light associated with the lighted display.
In another method for controlling the brightness of a display device, one or more user brightness selections are determined. A first signal is generated in response to the ambient light associated with a lighted display. A display luminance is selected from one or more luminance adjustment sequences. Each luminance adjustment sequence has multiple luminance values with constant ratio steps. The display luminance is provided to the lighted display. The display luminance is a fractional power function of the ambient light near with the lighted display. The fractional power function is adjusted by a constant luminance ratio offset based on the at least one user brightness selection.
Other systems, methods, features, and advantages of the invention will be or will become apparent to one skilled in the art upon examination of the following figures and detailed description. All such additional systems, methods, features, and advantages are intended to be included within this description, within the scope of the invention, and protected by the accompanying claims.